There is a pressing need for new regimens to treat existing cancers and prevent their recurrence. A recently developed strategy that shows great promise is the induction of tumor cell apoptosis (programmed cell death) through the triggering of death receptors expressed on tumor cell surfaces.
Cell apoptosis (programmed cell death) is a normal process through which cells of the body are induced to commit suicide when they are aged, damaged, or under attack by the immune system. For example, one mechanism for the elimination of virally infected, foreign, and tumor cells is the induction of apoptosis in these cells by attacking cytolytic T cells and other immune effector cells (Smyth et al., 2003; Bolitho et al., 2007). Apoptosis may have evolved as a mechanism to produce cell death without harming the host through such phenomena as the release of toxic oxidants and DNA that occurs in necrosis. In apoptosis, cells are disassembled in an orderly way, with the debris packaged in vesicles for uptake by scavenger cells, including antigen presenting cells (Ramachandran et al., 2000).
Apoptosis can be induced through two signaling pathways: the intrinsic pathway, which is activated by intracellular mitochondrial signals, or the extrinsic pathway, which is initiated through engagement of pro-apoptotic surface receptors (Duiker et al., 2006; Ashkenazi et al., 2008). The intrinsic pathway mediates apoptosis in response to intracellular changes such as DNA damage. This is the mode of cell death triggered by aging and by chemo- or radiotherapy induced damage. The extrinsic pathway is triggered by the activation of death receptors of the tumor necrosis factor receptor (TNFR) superfamily. Agonists of these death receptors induce them to transduce death signals into the cell interior. The superfamily includes families of receptors for TRAIL, Fas ligand, and TNF-alpha.
The TRAIL family of receptors contains the most promising targets for cancer therapy or prophylaxis. They are so named because they induce apoptosis when agonized by tumor necrosis factor-related apoptosis inducing ligand (TRAIL). Of the five known TRAIL receptors, DR4 (TRAIL-R1) and DR5 (TRAIL-R2) are therapeutic targets, because agonists of these receptors trigger cell death (Hampton, 2006; Cretney et al., 2007; Rowinsky 2005; Shi et al; 2005; Clancy et al., 2005). The other three TRAIL receptor family members are decoy receptors, which do not transmit death signals (DcR1; DcR2; OPG). The genes encoding DR4 and DR5 are highly homologous and likely arose from a common ancestral gene (Guan et al., 2001). In mice, only one agonist TRAIL receptor, DR5, has been identified. It is highly homologous in both structure and function to both human DR5 and human DR4 (Wu et al., 1999). DR4 and/or DR5 are expressed in both solid tumors and hematological malignancies and in some normal tissues such as hepatocytes, myocytes, glial tissue, bronchial and alveolar epithelium and activated lymphocytes.
Tumor cells tend to be far more susceptible than their normal counterparts to apoptosis induced by TRAIL receptor ligation. The mechanism that allows TRAIL to preferentially induce apoptosis in tumor cells, while sparing most normal cells, is not fully understood, but may be associated with the expression of common oncogenes, such as myc and ras that sensitize cancer cells to the extrinsic pathway of apoptosis (Ashkenazi et al., 2008). In addition, relative levels of TRAIL death receptors, decoy receptors and apoptosis inhibitors such as FLIP, IAP or XIAP also impact susceptibility (Duiker et al., 2006). Other death receptor families such as Fas are less tumor specific in occurrence and action, but may also be potential targets for therapy.
One strategy for exploiting TRAIL death receptors for therapeutic purposes is to administer recombinant TRAIL to a tumor host. Another is to administer antibodies to the TRAIL receptor, which bind to and agonize that receptor, essentially mimicking TRAIL. Ongoing and completed Phase I and II clinical trials using these reagents are showing clinically promising outcomes with little clinical toxicity and several instances of disease stabilization. Mapatumumab a DR4 agonist mAb, was administered at doses up to 20 mg/kg in a phase I trial (Hotte et al., 2008). The treatment was well tolerated and maximum tolerated dose was not reached. Of 41 patients with solid tumor, 12 showed stable disease with median duration of 3.5 months. Lexatumumab (HGS-ETR2), a DR5 agonist mAb, was tested in 37 patients and 10 mg/kg was the maximum tolerated dose when administered every 21 days up to 43 cycles (Hotte et al., 2008). Twelve patients had durable stable disease that lasted for ˜4.5 months. Further testing of both mAb is on-going in open trials.
Unfortunately, monoclonal antibody regimes, essentially “passive immunotherapy”, are impractical for the prolonged treatments required for chronic diseases like cancer. There are two main problems. First, clinical monoclonal antibodies are extremely expensive to manufacture. They are produced through recombinant or hybridoma technology, and must be purified to meet clinical safety standards. Once administered, they have a serum half life on the order of 11-18 days and typically must be administered at two week intervals (Tolcher et al., 2007). Second, exogenous antibodies have the possibility of provoking host immune responses to themselves. Effects of this response can range from rapid neutralization of the antibody to the induction of inflammatory disease (Abhinandan et al., 2007). Once an immune response against a monoclonal antibody is established, it permanently renders the antibody useless for therapy.
The problems of great expense and anti-antibody response limit or destroy the usefulness of recombinant TRAIL and exogenous anti-TRAIL receptor antibodies in cancer treatment. Therapeutic treatment cannot be maintained for the prolonged periods needed to eliminate existing tumor, and to render lifelong protection. Lifelong prophylaxis may be necessary to prevent the development of metastases after elimination of primary tumor, and also to prevent primary tumors in individuals at high genetic or environmental risk of cancer.
These drawbacks of treatment with exogenous TRAIL and anti-TRAIL receptor antibodies are avoided when anti-TRAIL death receptor antibodies are induced in the host by a vaccine against TRAIL receptors. The host's own immune system will continue to produce antibodies for many years, with no risk of an immune response against any portion of the antibody. The only cost is that of a course of vaccination. Limited serum half life is not a problem when the antibody is produced continuously. Indeed, antibody production may be enhanced when TRAIL death receptor-expressing tumor cells reappear, thanks to the specific memory property of B cell responses.
The main roadblock to the development of a therapeutic or prophylactic vaccine against host cell death receptors has been the phenomenon of tolerance, the immune system's safeguard against autoimmune disease. Death receptors are self antigens. The immune system generally becomes tolerant to self antigens early in life. T lymphocyte clones specifically reactive to self antigens are either deleted or anergized during thymic development, or are kept in check at the periphery, mainly by diverse populations of regulatory T cells (Treg). Especially important are natural Treg which develop in the thymus upon high affinity recognition of antigens in the thymic stroma (Colombo and Piconese, 2007). It is often impossible to define an antigen and immunization protocol that will break tolerance to a self antigen to achieve effective vaccination. This problem has defeated the development of many vaccines intended to induce immune response against tumor antigens (Wei et al, 2004). This is equally true of vaccines intended to induce antibodies, as helper T cell aid is essential for most B cell responses.
Another roadblock to the development of an agonist anti-death receptor vaccine is the need for a fully competent immune system that can meet the challenge of mounting a response to a self antigen. Cancer patients are often immunocompromised by their disease. Regulatory T cells play a role here too, as do tumor-induced myeloid suppressor cells and immunosuppressive factors secreted by tumors (Widen et al., 2008). Chemotherapy and radiation treatments also suppress response. Because of immunosuppression, many cancer patients cannot respond to self antigens, including many of the self antigens overexpressed or inappropriately expressed on tumor cells (Wei et al., 2004). Finally, the long lived antibody titers induced by effective vaccinations may bring out side effects of death receptor agonism which are not apparent in short term treatments.